The same thing happened to a few of mine recently. 3 up and died for no apparent reason and then 2 were laying on bottom, very rapid gill movement. One of them died and the other one I am treating with quick cure (malachite green and formalin). After about 4 or so days his gill movement is finally slowing down. I think its some kind of parasite - maybe gill flukes??
This is the one that passed away
This is Ferb who is still hanging on. I think he is actually doing better and still eating which is a good sign.
I recommend having some melafix and pimafix on hand if you start noiticing sysmptoms with future fish.
I wouldnt recommend them OR bettafix for bettas. Many people have claimed that the main ingredient - a tea tree oil - actually destroys their labyrenth organ which the betta uses to breathe air from the surface. If you do use it, I would use with care and do not overdose at least not for bettas. I would think its OK to use with fish that breathe oxyen via the water. I have used it with no ill effects but I also underdosed just in case. A better medication to have for bacterial infections would be either API Furan 2 OR Kannaplex. Furan 2 is easier to find but Kannaplex is a bit stronger and can usually only be found online via ebay or amazon.com
for parasite infections - there are a few out there. Jungle's Parasite clear may be the easiest to find as walmart carries it. There is also General Cure by API. For internal parasite infections - I make my own medicated fish food as I have been told the best way to treat it is by the fish consuming the medication via food. I just use a fizz tab and add in something like wardly pellets. Supposedly the better quality pellets wont soak up the medication as well..or so I have been told.
Quick Cure works well for ick, velvet, fungus and supposedly gill flukes.
If you look through the different medications sold on this site, they give a good description of each medicine they sell. Its organized via brand like Krodon, Api, Mardel, ect